Solid polymer electrolyte fuel cells electrochemically convert reactants, namely fuel (such as hydrogen) and oxidant (such as oxygen or air), to generate electric power. These cells generally employ a proton conducting polymer membrane electrolyte between two electrodes, namely a cathode and an anode. A structure comprising a proton conducting polymer membrane sandwiched between two electrodes is known as a membrane electrode assembly (MEA). MEAs in which the electrodes have been coated onto the membrane electrolyte to form a unitary structure are commercially available and are known as a catalyst coated membrane (CCM). In a typical fuel cell, flow field plates comprising numerous fluid distribution channels for the reactants are provided on either side of a MEA to distribute fuel and oxidant to the respective electrodes and to remove by-products of the electrochemical reactions taking place within the fuel cell. Water is the primary by-product in a cell operating on hydrogen and air reactants. Because the output voltage of a single cell is of order of 1V, a plurality of cells is usually stacked together in series for commercial applications. Fuel cell stacks can be further connected in arrays of interconnected stacks in series and/or parallel for use in automotive applications and the like.
Catalysts are used to enhance the rate of the electrochemical reactions which occur at the cell electrodes. Catalysts based on noble metals such as platinum are typically required in order to achieve acceptable reaction rates, particularly at the cathode side of the cell. To achieve the greatest catalytic activity per unit weight, the noble metal is generally disposed on a corrosion resistant support with an extremely high surface area, e.g. high surface area carbon particles. However, noble metal catalyst materials are relatively quite expensive. In order to make fuel cells economically viable for automotive and other applications, there is a need to reduce the amount of noble metal (the loading) used in such cells, while still maintaining similar power densities and efficiencies. This can be quite challenging.
One approach considered in the art is the use of certain noble metal/non-noble metal alloys which have demonstrated enhanced activity over the noble metals per se. For instance, alloys of Pt with base metals such as Co have demonstrated circa two-fold activity increases for the oxygen reduction reaction taking place at the cathode in the kinetic operating region (amounting to about a 20-40 mV gain). However, despite this kinetic advantage, such catalyst compositions suffer from relatively poor performance in the mass transport operating regime (i.e. at high power or high current densities). For instance, state-of-the-art commercial CCMs comprising PtCo alloy cathode catalysts with Pt loadings in the range of about 0.25-0.4 mg Pt/cm2) show good performance (about 2 times the mass activity) at low current densities but poor performance at high current densities (e.g. greater than about 1.5 A/cm2) relative to Pt catalysts on the same carbon support. Some of the advantages and disadvantages of such alloys as cathode catalysts are discussed for instance in “Effect of Particle Size of Platinum and Platinum-Cobalt Catalysts on Stability”; K. Matsutani et al., Platinum Metals Rev., 54 (4) 223-232 and “Activity benchmarks and requirements for Pt, Pt-alloy, and non-Pt oxygen reduction catalysts for PEMFCs”, H. Gasteiger et al., Applied Catalysis B: Environmental 56 (2005) 9-35.
Thus, neither the common noble metal catalysts nor their alloys seemed able to satisfy the desired performance requirements of many applications at both low and high current densities. Mixtures of various kinds may be considered but with an expectation of a performance compromise at both low and high current densities. So instead, alloy catalyst compositions, such as PtCo, are presently considered predominantly for stationary applications and are less attractive for automotive applications which require higher power density.
There is therefore a continuing need to obtain improved cathode catalysts and/or structures that provide desirable performance at both low and high current densities and while further reducing the amount of expensive noble metal required.